Historical significance and taxonomic status of Ischyrodon meriani (Pliosauridae) from the Middle Jurassic of Switzerland

Ischyrodon meriani is an obscure pliosaurid taxon established upon an exceptionally large tooth crown of a probable Callovian (Middle Jurassic) age that originates from Wölflinswil, Canton of Aargau, Switzerland. Despite being known for almost two centuries, the specimen remains poorly researched. Historically, I. meriani has been associated, or even considered conspecific, with Pliosaurus macromerus and Liopleurodon ferox. However, neither of the two hypotheses have been tested through detailed comparisons or using modern quantitative methods. Here, we redescribe the type of Ischyrodon meriani, illustrate it, and compare to teeth of thalassophonean pliosaurids, with special focus on Jurassic representatives of the clade. Multivariate analyses show close similarities to L. ferox but comparable structures to those of I. meriani, including a distinctive pattern of the apicobasal ridges, are also observable in some mid-Cretaceous brachauchenines from the ‘Polyptychodon’ assemblage of East and South East England. While it is likely that I. meriani represents a Liopleurodon-like taxon, or is indeed conspecific with L. ferox, which would make I. meriani the proper name for the species, any such taxonomic considerations are hindered by the fragmentary nature of the type specimens of both these taxa as well as limited knowledge of the dental variability within and between individual jaws of L. ferox. Currently, I. meriani is best treated as a nomen dubium. Finally, we discuss the potential implications of I. meriani being conspecific with L. ferox, and additionally provide a commentary on the taxonomic status of Liopleurodon.


INTRODUCTION
Ischyrodon meriani is a name pertaining to an obscure pliosaurid taxon, first mentioned in Hermann von Meyer's article titled ''Mittheilungen, an Prof. Bronn gerichtet'' (''Contributions to Prof. Bronn'') and published on July 26th, 1838, in Neues Jahrbuch für Mineralogie, Geognosie, Geologie und Petrefaktenkunde (von Meyer, 1838. The journal was edited at that time by K.C. von Leonhard and H.G. von Bronn and the contributions that were being published there basically represented letters informing the editors about newly discovered fossil specimens, which were often introduced as new taxa. However, neither in-depth descriptions nor illustrations were usually provided. Ischyrodon meriani was described based upon an exceptionally large (∼110 mm high) tooth crown (NMB L.D.37) originating from the Middle Jurassic of 'Wölffiswyl' (Wölflinswil in modern spelling) in Canton of Aargau, Switzerland. Von Meyer learned about the specimen, along with other fossil materials, from Peter Merian, professor of physics, chemistry, geology, and paleontology at the University of Basel (Switzerland). Merian considered the specimen to be a new taxon and also coined the name Ischyrodon, though he has never published it himself. Eventually, von Meyer (1838) introduced the name Ischyrodon Meriani [sic] for the specimen. Still, despite being mentioned in several papers and text books in subsequent years (von Meyer, 1841;Hoffmann, 1845;Geinitz, 1846;Giebel, 1847;Pictet, 1853), the tooth crown had not been described and illustrated until 18 years after the publication of the initial article in which the taxon was named (von Meyer, 1856a).
The taxonomy of I. meriani has always been challenging. Von Meyer (1841) noticed that the Ischyrodon tooth resembles the teeth of Thaumatosaurus from the Middle Jurassic of Neuffen (Germany). Similarities between the tooth crown of I. meriani and the teeth of Thaumatosaurus and Polyptychodon were further discussed in the full description of I. meriani by von Meyer (1856a). At that time, however, it still remained unclear as to what group these animals belonged to. Kiprijanow (1883) considered Ischyrodon to be closely related to Pliosaurus, and included it, together with Polyptychodon and Thaumatosaurus, within a new group of Sauropterygia which Kiprijanow (1883) called Thaumatosauria. Von Quenstedt (1885) listed Ischyrodon meriani among the Plesiosauri [sic] and highlighted similarities between the tooth crown and the teeth of Pliosaurus and Polyptychodon. Zittel (1887Zittel ( -1890 considered it similar to Pliosaurus, whereas Lydekker (1889) formally listed it as a synonym of Pliosaurus, noting it possibly represents P. macromerus. Lydekker (1889: 132) wrongly regarded Ischyrodon to be from the Upper Jurassic of Würtemberg [sic], in southern Germany, and stated that it ''presents all the characters of the teeth of the English Kimeridgian Pliosaurs'', and added that ''from its large size [it] may be provisionally referred to [Pliosaurus macromerus]''. Lydekker (1889: 132) concluded that, ''[i]f this reference [is] correct the name P. meriani should supersede P. macromerus''. Following Lydekker (1889), Ischyrodon was considered a synonym of Pliosaurus in several subsequent studies (De Stefani, 1903;von Huene, 1934;Wild, 1968). In turn, Tarlo (1960) was the first to discuss I. meriani as possibly being conspecific with Liopleurodon ferox. Such option was further considered or otherwise mentioned in Persson (1963), Geister (1998), Noè (2001, Barrientos-Lara, Fernández & Alvarado-Ortega (2015), and Madzia (2016).
Following the recent progress in the understanding of pliosaurid dental anatomy (e.g., Madzia, 2016;Foffa et al., 2018;Zverkov et al., 2018;Madzia, Sachs & Lindgren, 2019), we redescribe the type of I. meriani, illustrate it in detail, and compare with teeth of other thalassophonean pliosaurids. We further study the specimen through multivariate analyses aimed to explore the morphospace occupation of I. meriani among thalassophoneans and to assess previously mentioned similarities between the type of Ischyrodon and Pliosaurus (Lydekker, 1889) and Liopleurodon (Tarlo, 1960). Finally, we discuss potential implications of I. meriani being conspecific with L. ferox, and comment on the taxonomic status of Liopleurodon.

GEOLOGICAL SETTING
NMB L.D.37, the type of Ischyrodon meriani, has a series of labels mentioning the 'Eisenrogenstein' from 'Wölflinswyl'. In the following, we shortly discuss the geology and the geographic origin and then the stratigraphic position.

Regional geology and the geographic origin
'Wölflinswyl', now spelled Wölflinswil, is located in the Swiss canton Aargau near the villages of Frick and Herznach ( Fig. 1) and, on a broader scale, between the towns of Basel and Zürich. In this area, Triassic and Jurassic sediments crop out. Here, we focus on the Jurassic since predominantly Jurassic sediments are exposed in this area. In this part of Switzerland, the Jura mountains, the Mesozoic strata are strongly folded and faulted tectonically in the south ('Faltenjura'-folded Jura), whereas the area of interest lies in the 'Tafeljura' (table Jura), which was much less affected by the Alpidic orogeny (Herzog, 1956).
Both the tooth and a rock sample contained in the same box show that the matrix is extremely rich in iron. This suggests that the tooth comes from the iron ore that was mined from around 1,200 until 1967 in various quarries and underground mines (Hüsser, 1996;Hüsser, 2002). Some of these former mines are indicated in the map (Fig. 1). Since the labels accompanying the specimen provide Wöflinswil as the locality, we must assume a locality that is closer to that village than to the neighboring village of Herznach, where parts of the biggest and most recent mine is still accessible. The former mines and quarries of Hinter-Raibach and Dachseln lie closest to Wöflinswil. According to Hüsser (1996), mines on the Chornberg-a small mountain between Herznach and Wöflinswil-were documented already in 1772. This fits reasonably well with the possible times of mining around 1800 at Hinter-Raibach or Dachseln. Nevertheless, we cannot rule out that the tooth was found outside of the mines in the scree.

Multivariate analyses
In order to explore the morphospace occupation of NMB L.D.37 among thalassophonean pliosaurids, and assess previously mentioned similarities of the type of Ischyrodon to Pliosaurus (Lydekker, 1889) and Liopleurodon (Tarlo, 1960), we performed cluster and principal coordinates analyses. We used the dataset of Bastiaans et al. (2021), which is a recent version of the one first published by Zverkov et al. (2018), and added NMB L.D.37. No further modifications have been provided. See Supplemental Information I for the matrix. We used the same protocol as Zverkov et al. (2018) and Bastiaans et al. (2021). The analyses were carried out in the R statistical environment (RStudio Version 1.2.5033 R Studio Team, 2019). We applied a 50% completeness threshold to remove the influence of taxa which are based upon insufficiently complete or preserved specimens, scaled the data to equal variance and a mean of zero through subtraction of the mean value for each character and divided it by the standard deviation. Using the package cluster v2.1.0 we applied the Gower metric (Gower, 1971) to create a distance matrix. A cluster dendrogram was produced from the resulting matrix with the stats package, using the Ward.D2 method.
Then, we took the same matrix and used ape v5.3 (Paradis, Claude & Strimmer, 2004) to explore the dental morphospace occupation of I. meriani among the thalassophonean pliosaurids through a principal coordinates analysis. We applied the Gower metric and the Cailliez correction for negative eigenvalues. See Supplemental Information II and Supplemental Information III for the R code and extended results, respectively.

Terminology of tooth orientation and morphology
We follow the terminology of tooth orientation of Smith & Dodson (2003): apical, toward the apices of the tooth crown or the tooth root; basal, toward the cervix dentis; distal, away from the tip of the snout; labial, toward the lips; lingual, toward the tongue; mesial, toward the tip of the snout.

Results of multivariate analyses
The results of multivariate analyses are broadly congruent with those published by Zverkov et al. (2018) and Bastiaans et al. (2021) and we refer to these two studies for more detailed account of the results and interpretations thereof. The principal coordinates analysis (PCoA) placed NMB L.D.37 on the positive side of the first axis and the negative side of the second axis, in a close proximity to 'Polyptychodon' type 3 and Liopleurodon ferox (Fig. 4A). The cluster analysis placed NMB L.D.37 within the 'conical' morphogroup of the cluster dendrogram and grouped it with Liopleurodon ferox (Fig. 4B).

When was the name Ischyrodon meriani formally established?
Owing to the fact that the publication, in which the name I. meriani was used for the first time, did not include detailed description of the type tooth, it has not been clear whether von Meyer's (1838) contribution actually formally established the taxon name. For example, Tarlo (1960) stated that I. meriani remained a nomen nudum until von Meyer (1856a). Article 12 of the ICZN (International Commission on Zoological Nomenclature, 1999) specifies that ''[t]o be available, every new name published before 1931 must satisfy the provisions of Article 11 and must be accompanied by a description or a definition of the taxon that it denotes, or by an indication'' (Art. 12.1.) and that a vernacular name, locality, geological horizon, host, label, or specimen do not in itself constitute a description, definition, or indication (Art. 12.3.).
In the original study, von Meyer (1838) noted the provenance of the tooth crown and mentioned that he would like to name it ''I. Meriani'' (also providing the name Ischyrodon in the same sentence). The description provided by von Meyer (1838) is limited to a statement that the type tooth surely belonged to a giant reptile (''Riesen-Saurus'') and that only Mastodonsaurus had similarly massive teeth; further noting that the teeth of the two taxa are very different (see von Meyer, 1838: 414, 415). Albeit brief and vague, this wording may qualify as ''a description or a definition of the taxon that it denotes''. Owing to the absence of a universally followed set of rules that would govern the zoological nomenclature in the 1830s and the rather frequent use of the name I. meriani between 1838 and von Meyer's (1856a) much more detailed description (von Meyer, 1841;Hoffmann, 1845;Geinitz, 1846;Giebel, 1847;Pictet, 1853), indicating that the authors mentioning the name treated it as an established taxon name, we are inclined to consider von Meyer's (1838) contribution as the reference that established the name I. meriani as an available name in accordance with Article 12 of the ICZN. Such an interpretation renders I. meriani to be the historically earliest established taxon name of Pliosauridae.

Is Ischyrodon meriani conspecific with Liopleurodon ferox?
The morphology and enamel character distribution of NMB L.D.37, as assessed through comparisons to teeth of other thalassophonean pliosaurids and by means of our multivariate analyses, show close similarities with Liopleurodon ferox. Such results reopen the question of the potential conspecificity of the specimens assigned to the two taxa. Tarlo (1960) was the first to note that the teeth of I. meriani and L. ferox are very similar but added that ''[i]t is not certain from Meyer's figure whether this assignment is altogether justified'' (Tarlo, 1960: 166). Interestingly, Tarlo (1960) suggested that it was the name I. meriani that could be synonymized with L. ferox, rather than treating L. ferox as the potential junior synonym of I. meriani. Even though the material that is currently assigned to L. ferox is much better researched, much more complete, and taxonomically informative (see especially Andrews, 1913;Tarlo, 1960;Noè, 2001), it is I. meriani that would have nomenclatural priority (providing that I. meriani is declared diagnostic).
Still, despite that both these taxa originate from strata that are comparable in age, the fragmentary nature of the type tooth of I. meriani, combined with the observed similarities between the specimen and teeth of other thalassophoneans, even including some of the latest-diverging members of the clade (Madzia, 2016: Fig. 5), would make such taxonomic decisions unsubstantiated at present. We agree with Tarlo (1960), and some subsequent authors, such as Noè (2001: 27, 31), that there are good reasons to consider I. meriani and L. ferox to likely represent the same thalassophonean pliosaurid. However, the synonymization of the two names would have to be preceded with a morphometric and morphological assessment of the teeth of particular individuals assigned to L. ferox that would explore the dental variability within and between their jaws and reveal whether the tooth of I. meriani falls within that variability.

Is Ischyrodon meriani a diagnostic taxon?
Despite its fragmentary nature, the type tooth of Ischyrodon meriani shows enough morphological characters to make a detailed comparison to teeth of other thalassophonean pliosaurid taxa feasible. At present, it appears impossible to list features, either autapomorphies or a unique combination of characters, that would enable the taxon to be diagnosed. Therefore, I. meriani is currently a nomen dubium, and the type tooth is best treated as Thalassophonea indet.
Is Liopleurodon ferox a diagnostic taxon? Sauvage (1873) established Liopleurodon ferox based on a single isolated tooth crown (BHN 3R 197) originating from the upper Callovian (uppermost Middle Jurassic) strata of Le Wast, France. The description of the morphology of the holotype has been generally considered adequate to treat L. ferox as a diagnostic pliosaurid taxon and numerous additional specimens have been assigned to it or associated with it since its establishment (see especially Noè (2001) for detailed discussion of the history of L. ferox, specimens referred to the taxon, and their stratigraphic range and geographic distribution). Some of these specimens have originated from distant localities (e.g., Zverkov, Shmakov & Arkhangelsky, 2017) or are a few million years younger than the type tooth (e.g., Madzia, Březina & Calábková, 2018).
Owing to the rather broad geographic and stratigraphic distribution of the specimens assigned to or associated with L. ferox, it remains unclear whether these are in fact representatives of a single taxon (or clade), or whether the Liopleurodon dental morphology does not simply reflect a 'stage' in pliosaurid evolutionary history.
Additionally, it remains unclear whether the Liopleurodon lineage comprises one taxon, L. ferox, or two taxa, L. ferox and L. pachydeirus. Tarlo (1960) was convinced that L. pachydeirus, established as Pliosaurus pachydeirus by Seeley (1869), represents a diagnostic taxon that can be distinguished from L. ferox based on the morphology (coarseness) of the apicobasal ridges of the tooth crowns, and the proportions of cervical vertebrae and femora, citing two specimens, the type CAMSM J.46912 and NHMUK PV R 2446, as representatives of the taxon. In the most recent revision of Liopleurodon by Noè (2001), however, L. ferox and L. pachydeirus are considered synonyms. According to Noè (2001: 172), ''[t]he cervical vertebral characteristics utilised to distinguish Liopleurodon pachydeirus from Liopleurodon ferox (Tarlo, 1960) are non-diagnostic (based on Brown, 1981), and those of the teeth are here considered to represent individual variation'', though Noè (2001: 172) also notes that ''[t]he holotype material of the two species cannot be directly compared, as that of Liopleurodon ferox is a single tooth, and that of Liopleurodon pachydeirus is a string of 17 cervical vertebrae including an atlas-axis complex''. Unfortunately, Noè's (2001) PhD dissertation, though commonly referenced in the literature, remains formally unpublished and no detailed revision of Liopleurodon has been provided since the early 2000s.
Even if the differences observed between the specimens assigned to L. ferox and those considered to form L. pachydeirus represent individual variation, which indeed appears plausible, Liopleurodon should be still subjected to detailed reevaluation. As per Noè (2001: 142), the crown ornamentation of Liopleurodon is variable. It may therefore be needed that an ICZN petition is filed to replace BHN 3R 197, the type of L. ferox, with a different specimen, for example NHMUK PV R 3536 that is reasonably complete and has been commonly treated in comparative studies as a 'typical' representative of L. ferox.

CONCLUSIONS
Ischyrodon meriani is an obscure pliosaurid taxon based on a single tooth crown (NMB L.D.37) unearthed from the Callovian (Middle Jurassic) of Wölflinswil, Canton of Aargau, Switzerland. Despite its nearly two-century-long research history, the type of I. meriani is surprisingly poorly known. Previous studies have associated the material with Pliosaurus macromerus and Liopleurodon ferox, though neither of the two hypotheses have been thoroughly explored.
We have redescribed the type tooth of I. meriani, compared its morphology to that of other thalassophonean pliosaurids, and assessed its outer enamel structural elements through multivariate analyses. Our study supports close similarities of I. meriani with Liopleurodon ferox though comparable structures have also been observed in some latediverging brachauchenines. While we are inclined to consider I. meriani conspecific with L. ferox, the synonymization of both these names, which would make I. meriani the proper name for the taxon, would be unsubstantiated at present. Both, I. meriani and L. ferox are based on single tooth crowns whose morphologies do not appear to be unique enough to enable the taxa to be adequately diagnosed. We refrain from making far-reaching decisions regarding L. ferox; however, I. meriani is treated here as a nomen dubium.
Following the redescription of I. meriani, we comment on the taxonomic status of Liopleurodon ferox. We posit that a detailed reevaluation of Liopleurodon appears to be needed and replacement of its type specimen may be necessary to maintain the current understanding of the taxon.